The present invention relates generally to electroluminescent displays, and more particularly to a memory configuration for implementing gray shade electroluminescent displays.
As is known, electroluminescent displays or "ELDS" generally comprise a matrix of pixels, each of which is illuminated by a voltage potential between a row driver and column driver. The apparent brightness of a pixel can be varied by varying the refresh rate of that pixel. A pixel which is refreshed more frequently will appear brighter, while a pixel which is refreshed less frequently will appear dimmer. By varying the brightness of the different pixels in a display, a gray shade or gray scale image can be displayed.
Generally, a frame includes a brightness code for each pixel indicating a brightness level for that pixel in that frame. For example, the brightness code may comprise four bits of data indicating the brightness level for that pixel in that frame, with 0000 indicating that the pixel should be off, and 1111 indicating that the pixel should be illuminated at maximum brightness. In order to vary the brightness of a pixel utilizing a variable refresh rate, this brightness code must be converted to a refresh rate.
One way of utilizing a variable refresh rate among the pixels to implement brightness is to refresh the screen at a frequency which is multiple of the frame rate, refreshing each of the pixels a varying number of times for each frame. For example, for a frame rate of 60 htz, a refresh rate of 240 htz may be used. Thus, each pixel could be refreshed 0-4 times for a single frame, depending on the level of brightness indicated for that pixel. If a pixel is not refreshed during that frame, the pixel will be off. If the pixel is illuminated one time during the four refresh cycles, it will appear very dim. If the pixel is illuminated two or three times, it will appear brighter. If the pixel is illuminated four times in the four cycles, it will appear very bright.
One known configuration for implementing this technique is to decode the brightness information for each frame four times. During each refresh cycle for a frame, the decoder reads the brightness information for each pixel and stores a refresh command (a 0 or a 1) in a corresponding location for the pixel in a buffer. When all of the pixels have been decoded for a frame in a single refresh cycle, the buffer is copied to a second memory buffer, from which the column and row drivers are operated, illuminating or not illuminating each pixel as according to its associated refresh command. Over the four cycles, the decoder will send varying numbers of zeros and ones, depending upon the level of brightness indicated for that pixel in that frame. If the pixel is off, the decoder will send four zeros over the four refresh cycles. If the pixel is very bright, the decoder will send four ones over the four refresh cycles. If the pixel is of mid-brightness, the decoder will send alternating zeros and ones or a single one and three zeros or three ones and a single zero. Thus, over the four refresh cycles, the pixels will have different refresh rates, thereby providing an apparent brightness variation. However, this decoder is expensive, as it must decode all of the pixels in a frame at the refresh rate, in this example 240 htz. Further, the memory buffers are also expensive, since they must also read and write at the refresh rate.